The invention relates to a method for producing an electrode. More specifically, the invention relates to a method for producing an electrode using microscale or nanoscale materials obtained from hydrogen driven metallurgical reactions.
The miniaturization of electronic components has created widespread growth in the use of portable electronic devices such as cellular phones, pagers, video cameras, facsimile machines, portable stereophonic equipment, personal organizers and personal computers. The growing use of portable electronic equipment has created ever increasing demand for improved power sources that are safe, long-lasting, and are high energy density rechargeable batteries.
There is also an ongoing investigation into the manufacture of batteries suitable for use in electric motor vehicles or hybrid motor vehicles that can operate on electric and combustion power.
A battery or voltaic cell generally includes two chemicals or elements with differing electron-attracting capabilities that are immersed in an electrolytic solution and connected to one another through an external circuit. These two chemicals can be referred to as an electrochemical couple. The reaction that occurs between an electrochemical couple and a voltaic cell is a reduction-oxidization (redox) reaction.
The mechanism by which a battery generates an electric current typically involves the arrangement of chemicals in such a manner that electrons are released from one part of the battery via said redox reaction and made to flow through an external circuit or cell connection to another part of the battery. The element of the battery at which the electrons are released to the circuit is called the anode. During discharge, oxidation reactions occur at the anode. The element that receives the electrons from the circuit is known as the cathode, or the positive electrode. During discharge, reduction reactions occur at the cathode.
At rest, a voltaic cell exhibits a potential difference (voltage) between its two electrodes that is determined by the maximum amount of chemical energy available when an electron is transferred from one electrode to the other. The current that flows from the cell is determined by the resistance of the total circuit, including that of the cell itself. Further, a voltaic cell has a limited energy content, or capacity, that is generally given in ampere-hours and determined by the quantity of electrons that can be released at the anode and accepted at the cathode. When all of the chemical energy of the cell has been consumed (usually because the anode has been completely discharged) the operating voltage falls to zero and will not recover unless the battery can be recharged. The capacity of the cell is determined by the quantity of active ingredients in the electrode.
Presently, the most widely used rechargeable batteries are secondary batteries employing aqueous electrolytes, such as nickel/cadmium and nickel metal-hydride batteries. The half-cell reactions taking place in a nickel/metal-hydride cell may be written as follows:
Anode
MHx+xOHM+xH2O+xexe2x88x92xe2x80x83xe2x80x83[1]
Cathode
Ni(OOH)+H2O+exe2x88x92Ni(OH)2+OHxe2x88x92xe2x80x83xe2x80x83[2]
It is in effect a rocking chair type electrochemical cell in which hydrogen is transferred from one electrode to the other.
Nickel/metal-hydride cells have similar operating characteristics to nickel/cadmium cells, but the nickel/metal-hydride cells use a metal-hydride anode in place of cadmium.
At the anode of the nickel/metal-hydride cell, a reversible electrode oxidation reaction occurs with OHxe2x88x92 ions at the surface of the electrode. When the battery is charged, a corresponding reduction reaction occurs at the surface of the electrode in which hydrogen is absorbed into the metal producing a solid metal hydride and a hydroxide ion. The metal expands when absorbing the hydrogen and shrinks when releasing the hydrogen. The increase in volume during the hydriding reaction is a consequence of the volume of the absorbed hydrogen atoms.
The atomic volume of hydrogen in a metal, VH, is defined as the increase in the volume of the unit cell of the metal upon the insertion of one hydrogen atom. The expansion of the metal due to the absorption of hydrogen has been directly correlated to electrode corrosion. See Willems J. J. G. and Buschow K. H. J., J. Less-Common Metals, 129:13(1987).
There is also a corresponding contraction when hydrogen is removed. The anode is therefore subjected to volumetrically induced strains during charging and discharging cycles. This imposes great mechanical stress on the alloy which, consequently, breaks down into small particles. Furthermore, a large volume change in each charge and discharge cycle increases the flushing action of the electrolyte through the pores and micro-cracks of the electrode, thereby increasing the corrosion rate.
Lithium batteries have also been investigated vigorously as a battery that can ensure a high-energy density. Lithium batteries are prepared from one or more lithium electrochemical cells. Such cells typically include an anode of metallic lithium, a cathode, typically LiMeO2 (Mexe2x95x90Co, Ni or Mn), and an electrolyte interposed between separated positive and negative electrodes. The electrolyte typically comprises a salt of lithium dissolved in one or more solvents, typically non-aqueous (aprotic) organic solvents.
By convention, the lithium electrode of the cell is defined as the anode and the counter electrode is referred to as the cathode. During use of the cell, lithium ions (Li+) are transferred to the xe2x80x9canodexe2x80x9d on charging (in reality the Li electrode is acting as a cathode in the charging step). During discharge, lithium ions (Li+) are transferred from the Li (anode) to the now positive electrode (cathode). Upon subsequent charge and discharge, the lithium ions are transported between the electrodes. Cells having metallic lithium anode and complex metal oxide cathode are assembled in the charged state.
During discharge, lithium ions from the metallic anode are transported through the liquid electrolyte to the cathode. During charging, the flow of lithium ions is reversed and they are transferred from the xe2x80x9ccathodexe2x80x9d material through the electrolyte to the lithium xe2x80x9canodexe2x80x9d.
However, Li metal anodes have a serious disadvantage as they undergo undesirable morphological changes which not only affect cell performance, but can also constitute a severe safety hazard. The substitution of lithium alloys for metallic Li does not improve the situation as they are subject to severe strain due to the charging and discharging process. This is due to the large volume changes the bulk alloy undergoes when Li is inserted or removed. For example, in the case of tin, the expansion can be as high as 300% upon Li insertion. See K. D. Kepler et al. Electrochemical and Solid State Letters, 2(7): 307 (1999). Of course, there is a corresponding contraction upon the discharge of Li from the alloy. The stress incurred in charge and discharge of Li rapidly fractures the alloy into smaller particles. As this occurs the particles lose contact with one another and become electrically isolated and inactive. This is believed to be the source of the poor reversibility of such electrodes, which is reflected by rapid capacity decay.
Therefore, there is a need for electrochemical cells, such as lithium, nickel/metal hydride, etc., with electrodes that can better withstand such volumetrically induced strains.
In an effort to avoid the problems associated with metal anodes, the Sony Corporation introduced a lithium ion battery in approximately 1991 where the lithium metal anode was replaced by an anode made of a carbonaceous material. In this battery, lithium metal need not be present at any time. Lithium ions are transported back and forth between a cathode and a carbon intercalation anode. While cycle life and safety was considerably improved relative to Li metal anodes, a substantial penalty was incurred in battery capacity. Further, the battery must be fabricated with a cathode containing a large excess of Li, since it must provide enough Li in the first charge cycle to form the Li/C intercalation compound which is the active anode material. See P. G. Bruce, Royal Society, Chemical Communications, 1897 (1997). In addition, some Li is irreversibly absorbed by the carbon. These are considerable disadvantages, since the battery must undergo long, tedious and expensive operations at the factory before it can be used.
Recently, a new approach to the use of non-carbon anodes was described by Idota et al., Science, 276:1395 (1997), involving a composite oxide glass containing SnO. Initially it was thought that Li was inserted into the oxide in a mechanism similar to the Lixe2x80x94C intercalation process. However, Dahn et al., J. Electrochem. Soc., 144:2045 (1997) have shown that the initial reaction upon Li insertion is the formation of Li2O and metallic Sn. After the complete reduction of Sn, the introduction of additional Li can form a series of LixSn phases where x may vary from 1.71 to 4.4. These LixSn phases form extremely small grains ( less than 100 nm) embedded in an oxide matrix. In subsequent cycles, the Li/Sn alloy grains react reversibly with respect to Li removal and insertion. The presence of the inert oxide matrix apparently helps to physically stabilize the electrode upon cycling. The presence of the small, active, alloy grains in an inactive matrix helps to solve important problems with respect to the insertion of Li into bulk alloys, i.e., its large partial molar volume and sluggish reaction kinetics. Mao et al., Electrochemical and Solid State Ltrs., 2:3 (1999) refer to this class of Li anode materials as nanocomposites.
However, a serious problem with the in situ preparation of Li/Sn alloys is the large amount of Li converted to Li2O in the first charge cycle due to the reduction of tin oxide to tin by Li. This means that the battery must be assembled with the cathode containing enough excess Li to reduce SnO in the anode as well as supplying the disadvantage will be incurred where any Li alloy anode is prepared via the in situ reduction of a metal oxide.
While it is possible to produce Li alloys ex situ as finely divided particles by mechanical attrition or ball milling; such methods have several disadvantages. Water and oxygen contamination must be rigidly excluded from the process. Therefore, the process is carried out under an inert atmosphere. Mechanical milling can also introduce impurities into the alloy particles from the equipment used in physically reducing the particle size. The mechanical equipment involved tends to cumbersome to operate and maintain. Mechanical milling also creates plastic deformation of the alloy particles, which can undesirably alter the their metallurgical properties. Finally, such mechanical processes are not very reproducible.
Therefore, there is also a need for an electrode which avoids the problem of Li being converted to Li2O in the first charge cycle. Further, it is also desirable to start with an anode where the reversible LixSn phase is incorporated in the anode before battery assembly as this would reduce, or even eliminate, the need for the cathode to contain Li upon battery assembly.
In accordance with the present invention, a method is provided for producing an electrode. A metal is subjected to a hydrogen driven process in order to reduce the particle size of the metal. Preferred hydrogen driven processes are the hydriding-dehydriding (HD) process, hydriding-dehydriding-disproportionation-recombination (HDDR) process, and variations thereof including the DDR and DR processes. The hydrogen driven processes are used to provide various microscale or nanoscale materials, which can then be formed into an electrode by conventional means.
When utilizing the HD method, a metal capable of forming a reversible metal hydride is hydrided sufficient to at least partially convert the metal to a metal hydride. The metal hydride is then dehydrided sufficient to re-form the original metal. These steps are alternately repeated sufficient to physically reduce the metal to a microscale material. The metal is not changed chemically, but the size of its particles are reduced. The microscale metallic particles can them be formed into an electrode by conventional means.
When utilizing the conventional HDDR method, a metal alloy having at least one component capable of forming a stable metal hydride is hydrided to form a reversible metal hydride. The reversible metal hydride is then dehydrided. The metal alloy is then hydrided and dehydrided, alternately, to produce a microscale metal alloy. The microscale metal alloy is disproportionated at an elevated temperature to form a stable metal hydride and metal component. The stable metal hydride is dehydrided and, subsequently, the metal from the dehydrided stable metal hydride is recombined with the metal component to re-form the microscale metal alloy. The disproportionation and dehydriding/recombining steps are repeated, alternately, sufficient to physically reduce the microscale metal alloy to a nanoscale metal alloy powder. The metal alloy powder is then formed into an electrode by any conventional means.
In another embodiment, a metal component can be additionally present when dehydriding the metal alloy in the first step of the HDDR process. The presence of such a metal can result in the formation of a new alloy during the subsequent DR step, which can then undergo cyclic disproportionation-recombination reactions to reduce the alloy to the desired particle size.
In another embodiment, a metal oxide is additionally present when initially hydriding the metal alloy to form a reversible metal alloy hydride. Preferred metal oxide components are SnO or SiO2. The presence of a metal oxide produces a nanocomposite material, which can then be formed into an electrode. A metal fluoride, such as SnF2, can similarly be present instead of a metal oxide when initially hydriding the metal alloy so as to yield a nanocomposite material.
In a preferred embodiment of the invention, a DDR (dehydriding, disproportionation, recombination) process is used with a complex metal alloy hydride as the starting material. The complex metal alloy hydride having at least one component capable of forming a stable metal hydride is irreversibly dehydrided to form a partially dehydrided complex metal alloy hydride. The partially dehydrided complex metal alloy hydride is then irreversibly disproportionated into a stable metal hydride and metal component. The stable metal hydride is reversibly dehydrided. Subsequently, the metal from the dehydrided stable metal hydride is recombined with the metal component to form a microscale metal alloy. The microscale metal alloy is then disproportionated and reversibly dehydrided/recombined, alternately, sufficient to physically reduce the microscale metal alloy to a metal alloy powder. The metal alloy powder is then formed into an electrode by any known means.
Preferred complex metal alloy hydrides are those having a Group 1 or 2 metal, Al or B, and hydrogen. LiAlH4 is most preferred. When irreversibly dehydriding the complex metal alloy hydride in the first step of the DDR process, a metal component, carbon, or combination thereof can be additionally present. Also, it is preferred that the starting materials, such as the complex metal alloy hydride and any additional materials initially present, be in granular form when irreversibly dehydriding the complex metal alloy hydride in the first step of the process.
A metal oxide component can also be present when irreversibly dehydriding the complex metal alloy hydride. The presence of the metal oxide component initially in the process yields a nanocomposite material product, which is formed into an electrode. Preferred metal oxide components are SnO, SnO2, SiO2, a transition metal oxide, MgO, CaO, Al2O3, any metal oxide capable of being reduced by lithium, or a combination thereof. SiO2 is preferred. Similarly, a metal fluoride component, such as SnF2, can be additionally present instead of a metal oxide component when irreversibly dehydriding the complex metal alloy hydride in the first step of the process. The initial presence of a metal fluoride component produces a nanocomposite material, which is then formed into an electrode.
In another embodiment of the invention, a metal alloy is subjected to a DR process (disproportionation and recombination) to yield a metal alloy powder. The metal alloy is reversibly disproportionated to form a stable metal hydride and metal component. The stable metal hydride is dehydrided and, subsequently, the metal from the dehydrided metal hydride is recombined with the metal component to form a microscale metal alloy. The microscale metal alloy is then alternately disproportionated and the resulting stable metal hydride is dehydrided sufficient to reduce the microscale metal alloy into a metal alloy powder.
A preferred metal alloy for the DR process is LixSn, wherein x ranges between 0.57 and 4.4. A metal component can be additionally present when disproportionating the metal alloy in the first step. Since the HD steps are bypassed, it is preferred that the starting materials already be in granular form. A metal oxide component, such as SnO, can be initially present when reversibly disproportionating the metal alloy so as to yield a nanocomposite material. A metal fluoride can be initially present instead of a metal oxide in order to form a nanocomposite material.
In another embodiment of the invention, a stable binary metal hydride is dehydrided in the presence of an additional metal component to directly yield a metal alloy powder. The metal alloy powder is then formed into an electrode. LiH is a preferred binary hydride. Aluminum is a preferred metal component. In a preferred embodiment, the stable binary metal hydride is dehydrided in the presence of a metal oxide component to directly yield a nanocomposite material. Again, LiH is a preferred binary metal hydride. SnO is a preferred metal oxide component. The processes may not directly yield the desired alloy particle size. Thus, in both processes described above using a stable binary metal hydride as a starting material, it may be necessary to alternately disproportionate the resulting microscale metal alloy and dehydride the resulting metal hydride sufficient to physically reduce the microscale metal alloy to a nanoscale material.
In another embodiment, in the processes which yield a metal alloy powder, instead of forming the metal alloy powder into an electrode, the metal alloy powder formed by the hydrogen driven process can be partially oxidized to form a nanocomposite material. The nanocomposite material can then be formed into an electrode by known means.
In another embodiment, after the electrode is formed by the processes discussed above, lithium can be electrochemically introduced into the electrode if desired so as to yield an electrochemically reversible lithium alloy within the electrode.
The method of the invention avoids the disadvantages associated with known processes such as mechanical attrition or ball milling. The method of the invention provides further advantages due to the great flexibility of materials with which the method can be used. An almost inexhaustible number of alloys and materials can be treated, and alloy microstructures can be varied as a function of process parameters. The process can be precisely controlled and reproduced.
Because the process of the invention provides electrodes which include finely divided materials, fracture during charging and discharging cycles is reduced as the smaller particle size increases its capability of accommodating the strain of inserted lithium. Also, alloys with smaller grain size not only rely on the bulk metal grain to store Li, but also on the large surface area occurring at the grain boundaries. Thus, volumetrically induced strains are substantially reduced in the grain interior, again decreasing the tendency for particle fracture.
Also, the method of the invention for producing an electrode avoids the large amount of Li converted to Li2O in the first charge cycle because the metal oxide can be reduced ex situ. Additionally, the active Li alloy grains in the electrode are produced ex situ which significantly reduces the initial lithium content of the cathode in the assembled battery.